In recent years, thin film transistors formed using a semiconductor thin film (having a thickness of several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface (e.g., a glass substrate) have been attracting attentions. Thin film transistors are widely used for ICs (integrated circuits) and electronic devices such as electrooptical devices. In particular, thin film transistors are urgently developed as switching elements of image display devices typified by liquid crystal display devices, EL display devices and the like. In a liquid crystal display device (active matrix liquid crystal display device) which adopts a method in which display patterns are shown on a screen by driving a pixel electrode arranged in matrix with use of a switching element, specifically, a voltage is applied between a selected pixel electrode and a counter electrode arranged to be opposite the pixel electrode, optical modulation occurs in a liquid crystal layer provided between the pixel electrode and the counter electrode, whereby a display pattern is formed by the optical modulation and is recognized by a user.
Such active matrix liquid crystal display devices have been used more widely and there are growing demands for larger area of screens, higher definition, and higher aperture ratio. Further, higher reliability is required.
It is very important for such active matrix display devices to suppress off current which is one of thin film transistor characteristics. For example, if off current (leakage current flowing between a source electrode and a drain electrode in turning off) of a thin film transistor formed in a pixel portion is large, stable and excellent display is difficult. It is thought that one cause of off current in an inverted staggered thin film transistor (in particular, a channel etched thin film transistor) which is widely used for an active matrix display device is that current flows in a back channel. A variety of devices are made to prevent current from flowing in a back channel (for example, Reference 1: Japanese Published Patent Application No. H8-8440).
In addition, as a switching element of an image display device, a thin film transistor using microcrystalline semiconductor is known, as well as such a thin film transistor using amorphous semiconductor or such a thin film transistor using polycrystalline semiconductor film (for example, Reference 2: Japanese Published Patent Application No. H4-242724, Reference 3: Japanese Published Patent Application No. 2005-49832, Reference 4: U.S. Pat. No. 4,409,134, and Reference 5: U.S. Pat. No. 5,591,987).
As a method for manufacturing a thin film transistor using microcrystalline semiconductor, a technique is known in which an amorphous silicon film is formed over a gate insulating film, a metal film is formed over the amorphous silicon film, and the metal film is irradiated with a diode laser to modify the amorphous silicon film into a microcrystalline silicon film. With this manufacturing method, the metal film formed over the amorphous silicon film only converts light energy of the diode laser into thermal energy and is removed in a later step. That is, the amorphous silicon film is heated only by heat conducted from the metal film and the microcrystalline silicon film is formed by this heat (for example, see Reference 6: Troshiaki ARAI and others, SID '07 DIGEST, 2007, pp. 1370 to 1373).